Many chlorination processes for producing carboxylic acid chlorides by reacting various types of halogenating agents with carboxylic acids or anhydrides thereof have been heretofore proposed. For example, there are (A) a process using phosphorus pentachloride, as disclosed in J. Am. Chem. Soc., 67 (1954) p. 2239; (B) a process in which phosphorus trichloride is used, as disclosed in J. Chem. Soc., (1954) p. 2030; (C) a process in which phosphorus oxychloride is used, as disclosed in J. Jpn. Oil Chem. Soc. (Yukagaku), 10 (7) (1961) p.435; (D) a process using thionyl chloride, as disclosed in J. Chem. Soc., (1953) p.2117; and (E) a process in which phosgene is used, as disclosed in J. Chem. Soc., 31 (1954) p.151.
The use of phosgene is advantageous in producing high purity carboxylic acid chloride, because the gaseous by-products can be readily removed from the reaction system. Generally, however, the rate of reaction between phosgene and a carboxylic acid or an anhydride thereof is considerably low as compared with that of a reaction which involves a halogenated phosphorus compound. Accordingly, the use of a catalyst is indispensable to accelerate the reaction for producing carboxylic acid chloride in an industrially feasible process using phosgene. In this case, however, as described in detail in JP-A-2-6412 (the term "JP-A-" as referred to herein signifies a "unexamined published Japanese patent application"), the process suffers precipitation of solids and contamination of crude products due to the occurrence of strongly colored decomposed products from the catalysts.